Variants of enzymes and other proteins that result from the segregation of alleles at structural gene loci have been found to be very common in natural populations using gel electrophoresis to separate them. We will purify and do peptide analysis on the proteins from many isogenic lines to determine whether an electrophoretic class is genetically homogeneous or heterogeneous. We will also look for genetic heterogeneity within electrophoretic classes by heat denaturation, urea denaturation, iso-electric focussing, gradient gel electrophoresis and immunological techniques. Genetic variation in proteins of three species of aphids is investigated and the genetic variants grown on different host plants to determine whether the host plant environment exerts any selective force on different genotypes. An attempt to measure selection in nature on electrophoretic variants of enzymes is also being made in desert populations of Drosophila pseudoobscura, by perturbing the gene frequencies in isolated desert populations and then following the return, or lack of return, of the gene frequencies to the previous equilibrium. Finally, we will investigate genetic differences between species in the time of appearance of various enzymes during development and then in species crosses to determine what the genetic control of the difference may be.